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3oo                        FUELS  (GENERAL METHODS)

in presence of copper oxide and lead chromate, copper spirals also being
used. About 0-4 gram of substance, not too finely powdered, is used. At
the beginning of the combustion, it is well to heat moderately and in a
current of air rather than of oxygen ; when the volatile products are burnt
—this being easily judged from the aspect of the coke remaining in the
boat_the fixed carbon is burnt at a high temperature in a current of oxygen.
If the undried substance is employed, the moisture content must be allowed

for.

7.  Determination of the Nitrogen.—This is made on 0-75-1 gram
of the finely powdered sample by Kjeldahl's method (see Fertilisers, p. 123).

8.  Determination of the Oxygen.—This is calculated by difference,
the percentages of carbon, hydrogen, nitrogen, volatile sulphur, ash and
moisture being added and the sum subtracted from 100.

2. Determination of the Calorific Power

The calorific power of a fuel, is the quantity of heat generated by the
complete combustion of i gram of the fuel, expressed in .small calories.

The small calorie (cal.) is the amount of heat necessary to raise by i°C. (more
exactly from o° to i°) the temperature of i gram of water. Some refer the
calorific power to i kilo and use as unit of heat the large calorie (cal.), which is
the amount of heat required to raise by i° C. the temperature of i kilo of
water ; the numbers are the same in the two cases.

In some cases also the evaporative power of a fuel is calculated, this repre-
senting the number of kilos of water at o° which could be transformed into
aqueous vapour at 100° by the combustion of i kilo of the fuel. Since each
kilo of water requires 637 large calories (100 to bring it from o° to 100° and
537 to transform it into steam also at 100°), the evaporative power is obtained
by dividing the calorific power by 637.

The calorific power of a fuel may be calculated approximately from
the chemical composition, but it is best to determine it directly by calori-
metric methods. The calorific value is referred, according to circumstances,
to the fuel as such or simply dried, or to the pure fuel (moisture and ash
being deducted).

1. Calculated Calorific Power.—Formulae derived from that of
Dulong are used, but the results are only moderately satisfactory. Accord-
ing to Mahler,1 that to be preferred is the following, which gives, with most
coals, errors not exceeding 3%:

p = 81400 + 345ooH —3000(0 + N),

where C, H, 0 and N are the respective quantities of carbon, hydrogen,
oxygen and nitrogen contained in i gram of the pure fuel (moisture and
ash deducted) and p is the required calorific value, referred to the pure fuel.

By putting

0 + N = i -C-H,

the above formula simplifies to :

p = 111400 + 375ooH — 3000.

1 Etudes sitr les combustibles solides, liquides et gazeux (Paris, 1903), pp. 4 and 56.h water, filtered into another porcelain dish